The present invention relates to a vacuum switch especially suitable for high voltage operation and high repetition rate switching.
In recent years, development of high output lasers has been undertaken domestically and abroad and such lasers, including an excimer laser, a copper vapor laser, a TEMA-CO2 laser and a pulse driven CO2 laser, require a very high level of pulsed electrical input power of about several tens of GW within a period of time of several hundreds of ns. Typically, the laser is utilized for isotope separation of uranium atoms, photo-exciting chemical reaction and fine working of semiconductors. A hot-cathode gas-filled thyratron as shown in FIG. 10 is used with the laser as a switching device.
For example, the thyratron includes a gas-filled discharge tube in which an anode electrode 3, a cathode electrode 5 adapted to emit thermions and a grid electrode 6 are provided. When a positive voltage pulse is applied to the grid electrode 6 in order to change the potential at the grid electrode 6 from negative to positive, an glow discharge is initiated between the cathode and anode electrodes. With the thyratron activated, electric charge in a capacitor 18 is supplied to a laser discharge tube 20. The thyratron further includes a resistor 19, a heater 8 and a charging unit 22.
When used with a copper vapor laser for uranium isotope separation, the thyratron is required to be switched at several KHz. In operation of the thyratron, with the grid electrode 6 maintained at positive potential, thermions emitted from the cathode electrode 5 are attracted to the grid and anode electrodes 6 and 3 while colliding with hydrogen gas atoms, causing them to be ionized positively. The thus produced hydrogen ions (hereinafter referred to as plasma) cause partial discharge between the grid and cathode electrodes 6 and 5 and sympathetically with this partial discharge, partial discharge takes place also between the grid and anode electrodes 6 and 3, giving rise to ultimate glow discharge.
With the grid electrode applied with negative potential, the emission of thermions from the cathode electrode 2 is prevented and the plasma diffuses while colliding with the remaining hydrogen gas. This degrades the diffusion of the plasma. Consequently, plasma remains in the discharge space between the grid electrode 6 and each of the anode and cathode electrodes and hence insulation recovery is degraded, thus increasing the intervening time which precedes the next turn-on operation. Therefore, the conventional switch is disadvantageous in that it can not be used at high voltages and that it can not be switched at high repetition rates. The conventional switch also suffers from insufficient breakdown voltage in the event that the gas filled in the interior of the switch, such as hydrogen, is deteriorated. In addition, surge voltage concomitant with discharge is drawn to the grid electrode and the thyratron drive power supply is sometimes damaged.
To solve the above problems, JP-A-59-134517 proposes an arrangement as shown in FIG. 11 in which an electron beam is used in place of the grid electrode arranged between the anode and cathode electrodes, for performing switching operation. In this proposal, an electron beam 10A is emitted into a space between rod-like electrodes 9 and 9A in order that a gas such as argon gas for discharge control is ionized to initiate discharge. In this case, the electron beam is scattered by the discharge control gas filled in the space and disadvantageously, the discharge control becomes difficult to achieve. Further, because of the use of the gas for discharge control, the plasma diffusion is degraded in high repetition rate switching to cause insufficient breakdown voltage as in the case of the thyratron.